The present invention relates to an apparatus for producing a silicon semiconductor single crystal using the Czochralski method (referred to as a CZ method hereinafter), and a method for producing a silicon semiconductor single crystal with the apparatus.
Conventionally, in the growth of a silicon semiconductor single crystal using the CZ method, polysilicon is charged in a crucible provided in a growth furnace of an apparatus for producing a silicon semiconductor single crystal, the polysilicon is melted to silicon melt by heating the polysilicon with a heater provided around the crucible, and after a seed crystal is dipped into the melt, the seed crystal is pulled above the silicon melt while rotating it gently to grow a silicon semiconductor single crystal having a substantially cylindrical constant diameter portion. Then the pulled silicon semiconductor single crystal is cut and ground to leave the constant diameter portion, and becomes a silicon semiconductor wafer through a wafer shaping process. The thus obtained silicon semiconductor wafer is used as a semiconductor device substrate for fabricating an integrated circuit or the like on the surface layer of which is formed an electric circuit.
In the process of forming an electric circuit on the surface layer of the silicon semiconductor wafer, oxygen atoms contained in the silicon semiconductor wafer bond to silicon atoms to form oxide precipitates such as BMD (Bulk Micro Defect) inside the silicon semiconductor wafer. It is known that the oxide precipitates such as BMD capture (or getter) excess contamination atoms such as heavy metal atoms contaminated in the semiconductor device fabricating process to improve properties and yields of semiconductor devices. Therefore, by using a silicon semiconductor wafer substrate containing larger amounts of oxide precipitates such as BMD, it is possible to improve yields of semiconductor devices formed on a surface layer of the substrate.
The amount of oxide precipitates depends on a concentration of oxygen originally contained in the silicon semiconductor wafer as well as on thermal history of the silicon semiconductor wafer for a period from during the crystal growth up to just prior to the semiconductor device fabricating process. However, generally there is a standard for a concentration of oxygen contained in a silicon semiconductor wafer, which cannot be changed readily.
Also it is known that in the silicon semiconductor single crystal, even if distribution of an oxygen concentration in the direction of the growth axis is homogeneous, distribution of the amount of precipitated oxygen exists in a state where it is relatively large in the seed side of the grown crystal and is relatively small in the melt side. It is considered that this phenomenon is due to the distribution in the axial direction of thermal history in a relatively low temperature zone where nuclei of the oxide precipitates are formed and grow in the single crystal.
Then there is disclosed a technique for adjusting the thermal history to a desired value during a processing period from growing a silicon semiconductor single crystal up to manufacturing a silicon semiconductor wafer therefrom. For instance, JPA 83-120591 discloses a method of increasing oxygen precipitation by heating a silicon semiconductor single crystal during its growth to adjust the thermal history, and in JPA 90-263792, the method of annealing a silicon semiconductor single crystal after its growth and the like are examined.
In the method of heating a silicon semiconductor single crystal during its growth, however, there are required large scale reconstruction for installing a heating apparatus of heating a grown silicon semiconductor single crystal in a producing apparatus and a power for heating the grown crystal; the method cannot be regarded as an efficient method from the viewpoint of cost and operability. Further, a temperature balance during growth of a silicon semiconductor single crystal is forcibly changed, so that dislocations are created in the grown crystal, thereby the commercialization thereof being disadvantageously impossible.
On the other hand, in the method of annealing a silicon semiconductor single crystal after its growth, it is conceivable to anneal the silicon semiconductor single crystal in the ingot state or in the wafer state, but an expensive apparatus is required in either case, and in addition, running cost for the apparatus for annealing as described above is generally high, and therefore this method is inefficient in view of the production cost. Further, this method in which oxygen precipitation in a crystal is controlled by means of annealing has such troubles as contamination by heavy metals during the annealing process; so such problems persist in this method.
With the foregoing drawbacks of the prior art in view, it is an object of the present invention to provide an apparatus and a method for producing a silicon semiconductor single crystal which can stabilize and homogenize an amount of precipitated oxygen in the direction of the crystal growth axis when growing a silicon semiconductor single crystal.
To achieve the above described object, an apparatus for producing a silicon semiconductor single crystal according to the present invention resides in an apparatus for producing a silicon semiconductor single crystal by the Czochralski method which comprises a main growth furnace having a crucible retaining silicon melt disposed therein for growing a silicon semiconductor single crystal, and an upper growth furnace for housing therein and cooling the silicon semiconductor single crystal pulled from the silicon melt, wherein the upper growth furnace communicated to a ceiling section of the main growth furnace is provided with an upper insulating member for surrounding a pulled silicon semiconductor single crystal.
To make oxygen precipitated more in a silicon semiconductor single crystal, it is necessary to form therein nuclei for causing oxygen precipitation during crystal growth and to make the nuclei grown to large sizes. When a silicon semiconductor single crystal is subjected to heat treatment at a constant temperature, nuclei of oxide precipitates larger than the critical radius at the temperature grow to larger sizes, while those smaller than the critical radius are annihilated from inside of the silicon semiconductor single crystal. The critical radius of the nuclei of oxide precipitates becomes larger as the heat treatment temperature becomes higher. Therefore, to form BMD capable of gettering contaminants in a silicon semiconductor wafer, it is important to make the nuclei of oxide precipitates to sizes where the nuclei are not annihilated with heat treatment in the semiconductor device fabricating process. For that purpose, it is necessary to make the nuclei of oxide precipitates larger by adding heat treatment or thermal history more at a lower temperature than the heat treatment temperature in the semiconductor device fabricating process.
After a silicon semiconductor single crystal is formed in a growth furnace in a silicon semiconductor single crystal producing apparatus, the silicon semiconductor single crystal is pulled into the upper growth furnace and is allowed to cool down therein; therefore by adjusting the cooling rate at the low temperature section to a desired value, the silicon semiconductor single crystal is able to receive thermal history more to promote the formation of BMD.
To easily grow the silicon semiconductor single crystal having such quality as described above, the simple and best method is to arrange an upper insulating member for keeping warm a crystal pulled into the upper growth furnace such that the silicon semiconductor single crystal receives sufficiently thermal history at the low temperature section when the silicon semiconductor single crystal cools down. The upper insulating member may have a length almost similar to the full length of the upper growth furnace or may be arranged so as to keep warm at least about one twentieth of the full length of the upper growth furnace. When a length of the upper insulating member arranged in the upper growth furnace is less than one twentieth of the full length of the upper growth furnace, it is difficult to realize the sufficient keeping warm effect.
To sufficiently and suitably achieve the keeping warm effect for a silicon semiconductor single crystal at a low temperature area, a temperature inside the upper growth furnace communicated to the ceiling section of the growth furnace provided in the apparatus for producing a silicon semiconductor single crystal is 800xc2x0 C. or less, or the upper growth furnace is arranged such that, even when a length of the upper insulating member is minimal, a temperature section of from 400xc2x0 C. to 650xc2x0 C. is kept warm; by adjusting the upper insulating member in such a way that thermal history of the silicon semiconductor single crystal in the above described temperature area become longer, it is possible to make the amount of precipitated oxygen larger and also to ensure stable oxygen precipitation along the full length of a crystal.
Especially, with an apparatus for producing a silicon semiconductor single crystal having the construction of the inventive apparatus, when growing a silicon semiconductor single crystal, as a first half portion of the crystal corresponding to the seed crystal side passes through the insulating member arranged in the upper growth furnace during growth of a second half portion of the crystal, it can receive sufficiently thermal history in the low temperature section; although the second half portion of the silicon semiconductor single crystal does not pass through the insulating member during growth of the single crystal, when the silicon semiconductor single crystal is pulled into the upper growth furnace and cooled down to such a low temperature as the silicon semiconductor single crystal can be taken out, the second half portion is surrounded by the insulating member of the upper growth furnace, so that the second half portion can receive sufficiently thermal history in the low temperature section like the first halt portion of the silicon semiconductor single crystal even after the crystal is separated from the silicon melt.
By taking the above described countermeasures, although there is generated a slight difference between the first half potion and the second half portion of the crystal in terms of the time when the silicon semiconductor single crystal is kept warm at a low temperature due to operating conditions for growing the silicon semiconductor single crystal or other reasons, the difference in an amount of precipitated oxygen between the first half portion and the second half portion of the crystal is substantially smaller as compared to that when the insulating member is not used; it is possible to obtain a crystal with the amount of precipitated oxygen homogenized along the full length of the crystal straight body.
Further, an amount of precipitated oxygen generated in a silicon semiconductor single crystal depends largely on the duration of thermal history in the low temperature section when the crystal is cooled down; by adjusting a length of the upper insulating member arranged in the upper growth furnace of the producing apparatus to a kind or a quality of a silicon semiconductor single crystal to be grown, it is possible to more efficiently grow a silicon semiconductor single crystal having a desired amount of precipitated oxygen. An upper insulating member in the upper growth furnace may be exchanged with one having a different length, whenever growing a silicon semiconductor single crystal, according to a crystal kind such as a diameter and a length of the silicon semiconductor single crystal to be pulled or an oxygen concentration in the crystal. Also it is preferable to provide a plurality of upper insulating members themselves piled up in the direction of the crystal growth axis in the upper growth furnace, so that the number of the upper insulating members can be changed for changing a temperature range of a lower temperature section of the crystal to be kept warm according to a desired amount of precipitated oxygen in the silicon semiconductor single crystal.
With the above described producing apparatus, it is possible to adjust suitably a temperature range width of a low temperature section of a crystal to be kept warm by changing a length of the upper insulating member arranged in the upper growth furnace; it is possible to control an amount of precipitated oxygen in the silicon semiconductor single crystal to be pulled to a desired value.
On the other hand, the upper insulating member arranged in the upper growth furnace for keeping warm the silicon semiconductor single crystal is exposed to a high temperature ranging from several hundreds c to about 800xc2x0 C. even in the upper growth furnace; it is preferable to use an upper insulating member made of the materials shaped from the same carbon fiber as the insulating member for a heater or the like provided in the growth furnace for growing a silicon semiconductor single crystal. Further, in order to prevent impurities or the like out of the insulating member from flying into the growth furnace, it is desirable to cover a surface of the upper insulating member with high purity graphite materials or high purity graphite materials with a surface coated with a film of pyrolytic carbon or silicon carbide, or with metallic materials containing a metal selected from the group consisting of iron, nickel, chromium, copper, titanium, tungsten, and molybdenum as the main ingredient.
With the apparatus described above, as an amount of precipitated oxygen in a silicon semiconductor single crystal in the direction of the crystal growth axis can be stabilized almost uniformly, there is reduced variations in an amount of precipitated oxygen between individual products generated when silicon semiconductor wafers obtained from respective portions of a crystal are subjected to heat treatment of some kind such as device simulation in the next process; therefore it is possible to stabilize device quantity and production yield.
A first aspect of the method for producing a silicon semiconductor single crystal according to the present invention is to produce the silicon semiconductor single crystal by the use of the apparatus for producing a silicon semiconductor single crystal according to the present invention.
A second aspect of the method for producing a silicon semiconductor single crystal according to the present invention resides in a method for producing a silicon semiconductor single crystal by the Czochralski method, wherein the silicon semiconductor single crystal pulled from a crucible is grown keeping warm a portion thereof with a temperature of 800xc2x0 C. or less without heating it from the outside.